This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Background: The effects of recipient status on transgenic mouse production. Pseudo-pregnant female mice (recipients) serve as surrogate mothers to carry micro-injected embryos to term. Yet the potential impact of the interval between the pairing of females with vasectomized males and subsequent embryo transfer on transgenic mouse production has generally received little attention. Therefore, a retrospective study was conducted to assess the effects of this interval on pregnancy rate and efficiency of producing transgenic pups using ICR mice. Methods: Data were collected over a five year period at the Ohio State University Transgenic Mouse Core Facility. We compared transgenic mouse production using females paired with vasectomized males on the morning of embryo transfer surgery (0 dpc) with females paired the day prior to surgery (0.5 dpc). Results/Discussion: Conception frequency was approximately 85% in both cases (1197/1428 = 83.8% for 0.5 dpc;28/33 = 84.8% for 0 dpc). Similarly, the proportion of transgenic pups within litters was not affected by the timing at which mating had occurred (24/151 (16%) vs 709/5363(13%) for same-day vs previous-day matings, respectively). Thus, the efficiency of producing pregnancies as well as transgenic offspring does not appear to depend on whether potential recipients are paired and plugged on the day of transfer as opposed to the more traditional method of placing them with males on the preceding day. In contrast, when vaginal plugs were seen in the morning but could no longer be detected by the time of embryo transfer (mostly late afternoon), the pregnancy rate decreased to 54.2% (13/24) with a concomitant reduction in the appearance of transgenic pups (1/36 (2.8%)). The principal conclusion from this study is that the pseudo-pregnant recipients obtained by pairing of females with vasectomized males on the morning prior to embryo transfer surgery can be used without reservation, while the recipients lost the vaginal plugs should be used with caution.